1. Field of the Invention
The present invention relates to a position detection apparatus for use in controlling position of a movable portion of a machine tool or industrial apparatus and in particular, to a position detection apparatus for detecting a position of a movable portion performing a linear movement.
2. Description of the Prior Art
Conventionally, a so-called position detection apparatus has been used for performing a position control of a movable portion which performs a linear motion.
The position detection apparatus generally includes a main body and a head slider provided in the main body in such a manner that the head slider can move in a linear motion.
In the main body, there is provided, a recording medium shaped, for example, in a round bar, and called "scale". On this scale, a position signal is recorded along the longitudinal direction of the scale. Moreover, the head slider has a detection head for detecting the position signal recorded on the scale. This detection head is, for example, provided at a position facing to this position signal and moves in the longitudinal direction of the scale together with the linear movement of the head slider.
In such a scale apparatus constituted by the main body and the head slider is mounted on a reference block and a movable block of a machine tool performing a relative linear movement. That is, the main body or the head slider is fixed on the reference block, and the other is fixed on the movable block. Here, the main body is mounted in such a manner that the longitudinal direction of the scale is in parallel to the movement of the movable block. Accordingly, in this scale apparatus, as the movable block moves in a linear motion, the relative position between this main block and the head slider is changed.
In this position detection apparatus having the aforementioned configuration, the detection head provided in the slider head detects the position signal changing according to the change of the relative position, so as to control the position of the movable position.
In the position detection apparatus, a scale signal and an origin signal are recorded as the position signal in the longitudinal direction of the scale.
The scale signal has a pit and mark successively repeated at a predetermined interval in the longitudinal direction of the scale. For example, the scale signal is a magnetic signal in which polarity is inversed at a predetermined recording wavelength. By using the scale having such a scale signal, it is possible to linearly detect a change of relative movement position between the main body and the slider, so as to continuously control the relative position between the reference block and the movable block.
The origin signal is a signal in which at least one pit or mark is discretely recorded in the longitudinal direction of the scale. For example, one-wavelength magnetic signal is recorded at a predetermined one position in the longitudinal direction of the scale. By using the scale having such an origin signal, the position detection apparatus can detect a reference position such as an initial setting position and an origin position of the main body and the head slider, so as to set the relative position of the reference block and the movable block at the reference position.
In a machine tool or the like performing an NC (numerical control) feed back, it is necessary to control a relative movement amount of the movable block as well as to move the movable block to a reference position. For this, the machine tool performing such an NC feed back requires a position detection apparatus having a built-in scale on which two signals, i.e., a scale signal and an origin signal are recorded.
The position detection apparatus having a built-in scale having a scale signal and an origin signal recorded includes a detection head for detecting the scale signal and a detection head for detecting the origin signal. Each of the heads detects only a corresponding signal for supply to a control apparatus for driving-controlling the machine tool.
However, in such a position detection apparatus, when two signals of a scale signal and an origin signal are recorded on one scale, a crosstalk is generated between the two signals, deteriorating signals detected from the respective detection heads.
As the built-in scale becomes smaller for reduction of the size of the position detection apparatus, the crosstalk between the two signals has a greater effect, making it difficult to eliminate the effect.